Revert "[lldb] Refactor SBFileSpec::GetDirectory"

This reverts commit 2bea2d7b.

It introduced following failures on buildbot lldb-aarch64-windows:

lldb-api :: functionalities/process_save_core/TestProcessSaveCore.py
lldb-api :: python_api/symbol-context/TestSymbolContext.py

Differential Revision: https://reviews.llvm.org/D149625